1. Field of the Invention
The present general inventive concept relates to an inkjet printhead, and more particularly, to a thermal inkjet printhead having a low resistance wire.
2. Description of the Related Art
An inkjet printhead is a device for printing a predetermined color image by ejecting minute droplets of printing ink on a desired point of a printing paper. Inkjet printheads can be generally classified as to two types according to the ejection mechanism of ink droplets. A first type is a thermal inkjet printhead that ejects ink droplets using expansion force of ink bubbles that are created using a heat source, and a second type is a piezoelectric inkjet printhead that ejects inkjet droplets using a pressure created by the deformation of a piezoelectric element.
The ejection mechanism of ink droplets from the thermal inkjet printhead will be described in detail. When a pulse type current is applied to a heater composed of heating resistors, ink around the heater is instantly heated to approximately 300° C. due to the heat generated by the heater. Thus, the ink boils and thus, ink bubbles are generated. Then, the ink bubbles apply pressure to the ink filled in an ink chamber by expanding. As a result, ink near nozzles is ejected to the outside from the ink chamber through the nozzles in a droplet state.
FIG. 1 is a cross-sectional view illustrating a conventional thermal inkjet printhead. Referring to FIG. 1, the conventional thermal inkjet printhead includes a substrate 10 on which a plurality of material layers are formed, a chamber layer 20 stacked on the plurality of material layers, and a nozzle layer 30 stacked on the chamber layer 20. A plurality of ink chambers 22, in which ink that to be ejected is filled, are formed in the chamber layer 20. A plurality of nozzles 32 through which the ink is ejected are formed in the nozzle layer 30. An ink feed hole 11 for supplying ink to the ink chambers 22 is formed in the substrate 10. An insulating layer 12 for insulating a plurality of heaters 14 from the substrate 10 is formed on the substrate 10. The heaters 14 are formed on the insulating layer 12 to generate ink bubbles by heating the ink. Individual wires 16 that are electrically connected to the plurality of heaters 14 are formed on the heaters 14. The heaters 14 are formed of a heating resistor, for example, an alloy of tantalum-aluminum, tantalum-nitride, titanium-nitride, or tungsten-silicide. In FIG. 1, reference numeral 18 indicates a passivation layer for protecting the heaters 14 and individual wires 16, and reference numeral 19 indicates an anti-cavitation layer for protecting the plurality of heaters 14 from a cavitation force generated when ink bubbles disappear.
Although not illustrated in FIG. 1, a plurality of bonding pads 50 (as illustrated in FIGS. 2A and 2B) to which external voltages to drive the nozzles 32 are applied, and common wires 45 (as illustrated in FIGS. 2A and 2B) that are electrically connected to the bonding pads 50 are formed on the conventional thermal inkjet printhead. The individual wires 16 are electrically connected in parallel to each of the common wires 45. Each of the individual wires 16 is a metal layer having high electrical conductivity, and each of the common wires 45 is a metal layer having high electrical conductivity.
FIG. 2A is an equivalent circuit of the conventional thermal inkjet printhead when one of the nozzles 32 is driven. FIG. 2B is an equivalent circuit of the conventional thermal inkjet printhead when a plurality of the nozzles 32 are driven simultaneously. In FIGS. 2A and 2B, reference numeral 37 indicates a field effect transistor (FET) 37 for switching the operation of the nozzles 32. Vcc indicates an external voltage that is applied to the bonding pad 50, and Rcm, Rgm, Rheater and RFET indicate resistances of the common wire 45, the individual wire, the heater 14, and the FET 37, respectively.
Referring to FIG. 2A, when one of the nozzles 32 of the conventional thermal inkjet printhead is driven, a majority of power supplied from the outside can be used by one of the corresponding heaters 14 since the resistance thereof is generally much larger than that of the individual wire 16 and that of the common wire 45. However, as depicted in FIG. 2B, when the plurality of nozzles 32 are driven simultaneously, and since resistances related to the nozzles 32 are connected in parallel, a sum of the resistances is very small. As a result, an influence of the resistances of the individual wires 16 and the common wires 45, (in particular, the common wires 45), is very large, and accordingly, power supplied to the heaters 14 is reduced. To compensate for the reduced power to the heaters 14, an external voltage Vcc that is applied to the bonding pads 50 must be increased. Also, as the number of nozzles 32 that are driven simultaneously increases, the external voltage Vcc that is applied to the bonding pads 50 must be further increased. However, the number of nozzles 32 driven simultaneously varies. Therefore, if the external voltage Vcc increases when one of the nozzles 32 is driven, an excessive voltage Vcc is applied to one of the corresponding heaters 14, thereby reducing an operational lifetime of one of the corresponding heaters 14.
To avoid the above-described problem, it is necessary to reduce resistances of the individual wires 16 and the common wires 45, and, in order to do so, thicknesses of the individual wires 16 and the common wires 45 must be increased. However, when the thickness of the individual wires 16 is increased, it is difficult to precisely form the heaters 14 in a desired shape by patterning of the thick individual wires 16.